(1)
∠7 + ∠8 = 180° ( straight angle )
∠7 = 180° - ∠8 = 180° - 113° = 67°
(2)
∠C = 180° - 120° = 60° ( straight angle )
The sum of the angles in a triangle = 180°, hence
3y + 12 + 4y + 3 + 60 = 180
7y + 75 = 180 ( subtract 75 from both sides )
7y = 105 ( divide both sides by 7 )
y = 15
hence ∠B = ( 3 × 15 ) + 12 = 45 + 12 = 57°
(3)
∠C = 180° - 113° = 67°
The sum of the angles in a triangle = 180°, hence
3y + 10 + 4y - 2 + 67 = 180
7y + 75 = 180 ( subtract 75 from both sides )
7y = 105 ( divide both sides by 7 )
y = 15
hence ∠A = ( 4 × 15 ) - 2 = 60 - 2 = 58°
